Method of controlling illumination field to reduce line width variation

ABSTRACT

Blades pivotally attached together linked to push rods and inserted into an illumination field, energy or flux. The blades extend longitudinally along the length of a rectangular illumination field or slit used to image a reticle onto a photosensitive substrate. The blades controllably adjust the width of the rectangular illumination field to modify the illumination intensity or energy provided to a photosensitive substrate. The illumination field is scanned across the photosensitive substrate to expose it with the image of a reticle. The blades are dynamically controlled during the scanning exposure to adjust the illumination intensity or energy in a predetermined way. The resulting selective change in exposure dose corrects local area of line width variance. Various errors in pattern reproduction using a photolithographic system are relatively easily corrected. This is particularly advantageous in a scanning lithography system used in the manufacture of semiconductors.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/023,407 filed Feb. 12, 1998 which is a continuation-in-part of U.S.application Ser. No. 08/829,099 filed Mar. 31, 1997.

FIELD OF THE INVENTION

The present invention relates generally to illuminating a reticle foruse in lithography as used in manufacturing semiconductor devices, andparticularly to dynamically adjusting the illumination field forproviding a desired exposure to control and reduce line width variationover local areas.

BACKGROUND OF THE INVENTION

In the process of semiconductor manufacturing, lithography orphotolithography is typically used to project light through a reticleexposing a silicon wafer coated with photosensitive resist in selectregions to define circuit elements. An illumination system has been usedin step-and-scan photolithography equipment sold under the trademarkMICRASCAN by SVG Lithography Systems, Inc. Wilton, Conn. In thisphotolithography equipment, the reticle and the wafer move withdifferent speeds. The different speeds having a ratio equal to themagnification of the projection optics. A rectangular or slit fielddefined by the illumination system is scanned over the reticle and overthe wafer. A vertical field delimiter frames the vertical field height,and horizontal framing blades frame the horizontal field width. It isdesirable to have as uniform an exposure field as possible. Theillumination level is the integral, in the scan direction, of theillumination on the wafer. Often the illumination is not uniform enough.To obtain a uniform exposure or a constant level of illumination fromthe top to bottom longitudinally along the rectangular exposed field, anadjustable slit is often required. In the past adjustable slits havebeen used that employed a line of nails or projections perpendicular tothe illumination beam. Individual nails or projections would be pushedinto the illumination beam to make more uniform the illumination levelor energy. Additionally, metal strips placed at an angle to theillumination beam would be bent or warped by rods thereby adjusting andmaking more uniform the illumination level or energy. One type ofcompliant member or adjustable slit is disclosed in U.S. Pat. No.4,516,852 entitled "Method and Apparatus for Measuring IntensityVariations in a Light Source", issuing to Liu et al on May 14, 1985.Therein disclosed is an arcuate slit that is adjusted with a deformableband. While these prior adjustable slit devices have been helpful inproviding a more uniform illumination field the ever increasing demandsplaced on lithography in reducing feature size of semiconductor devicesand increasing yield require an even more uniform illumination field.

SUMMARY OF THE INVENTION

The present invention is a device for adjusting a rectangularillumination field or slit for providing a uniform or desiredillumination field used in scanning lithography. A plurality of bladesare coupled or linked together so as to form a movable edge along alength of the rectangular illumination field. The ends of each blade areattached by a pivot pin to a link. The link is attached by a pivot pinto a push rod. The link may be a rigid link or a flexure. The push rodsare independently adjustable causing the blades to be controllablyinserted into and out of the rectangular illumination field. The edgecorners of the blades have a radius equal to the distance of the pivotpin to the edge of the blade. The present invention also includes amethod of providing a predetermined exposure dose along the longitudinallength of the illumination filed depending upon the line width of afeature to be imaged. In another embodiment of the present invention thewidth of the illumination field along a longitudinal length is adjusteddynamically or during scanning of the illumination field over aphotosensitive substrate or wafer. A corrected exposure dose iscalculated based upon stored information of the photolithographic systemor tool signature, the particular reticle, and the resist responsefunction. The width of the illumination field is adjusted to obtain thecorrected exposure dose.

Accordingly, it is an object of the present invention to make moreuniform a rectangular illumination filed.

It is another object of the present invention to provide a constantillumination flux along a longitudinal length of the rectangularillumination field.

It is an object of another embodiment of the present invention tolocally adjust the illumination flux or energy to compensate for localareas of line width variance.

It is an advantage of the present invention that a smoothly continuousadjustment is made to the rectangular illumination field.

It is another advantage of the present invention that adjustments mayeasily be made to the rectangular illumination filed.

It is another advantage of the present invention that a constant linewidth to exposure ratio can be maintained improving system performance.

It is an advantage of another embodiment of the present invention thatillumination intensity or energy can be varied at different locationsalong an illumination field continuously during scanning.

It is a feature of the present invention that blades are coupledtogether by pivots forming an adjustable edge.

It is another feature of the present invention that a link is usedbetween the blade and a push rod.

It is a feature of an embodiment of the present invention that exposuredose is calculated and controlled by an illumination adjuster.

These and other objects, advantages, and features will become moreapparent in view of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 pictorially illustrates the illumination profile of a rectangularillumination field and the application of the present invention.

FIG. 2 graphically illustrates illumination energy.

FIG. 3 schematically illustrates an embodiment of the present invention.

FIG. 4 schematically illustrates the movement of the blades of thepresent invention.

FIG. 5A is a partial cross section of a portion of an embodiment of thepresent invention.

FIG. 5B is a partial cross section of a portion of the inventionillustrated in FIG. 5A rotated ninety degrees.

FIG. 6 is a perspective view of the present invention.

FIG. 7 is a block diagram illustrating the method steps of the presentinvention.

FIG. 8A is a schematic illustration of a reticle having a verticalfeature.

FIG. 8B is a schematic illustration of a reticle having a horizontalfeature.

FIG. 9A is a schematic illustration of a reticle having a first feature.

FIG. 9B schematically illustrates a reticle having a second featureorthogonal to the first feature illustrated in FIG. 9A.

FIG. 10 is a cross section schematically illustrating lineslithographically produced on a substrate.

FIG. 11 schematically illustrates in plan view a portion of alithographically produced line.

FIG. 12 is a block diagram illustrating the method steps of anotherembodiment of the present invention.

FIG. 13 is a schematic drawing illustrating the dynamically adjustableillumination field of another embodiment of the present invention.

FIG. 14 is a schematic plan view illustrating local areas of line widthvariance.

FIG. 15 is a graph illustrating the illumination intensity or energyprovided to a photosensitive substrate.

FIG. 16 is a graph illustrating a positive resist response function.

FIG. 17 is block diagram illustrating the method steps of an embodimentof the present invention.

FIG. 18 is schematic diagram illustrating a photolithographic system ortool incorporating an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an illumination profile 10 produced by anillumination system, not illustrated. A rectangular illumination fieldor slit 12 is formed. The illumination field 12 has a length along the Yaxis and a width along the X axis. Waveform 14 illustrates the intensitydistribution along the X direction or width of the rectangularillumination field 12. The illumination profile 10 may havenon-uniformities. Waveform 16 illustrates a non-uniformity. Thisnon-uniformity may result in uneven exposure of a photosensitive resistcovered substrate, such as a wafer which may result in poor quality orreduced yield. The present invention is illustrated generally as anadjustable slit device 20. The adjustable slit device 20 has a pluralityof adjustable blades that are selectively inserted into the illuminationprofile 10 along a longitudinal length of the rectangular illuminationfield or slit 12. The illumination energy or flux of the rectangularillumination field is thereby adjusted correcting or making more uniformthe illumination energy or flux along the longitudinal length of therectangular illumination field. Therefore, when the rectangularillumination field 12 is scanned in the X direction, indicated by arrow18, a desired more uniform exposure is obtained.

FIG. 2 graphically illustrates the improved more uniform illuminationenergy or flux achievable with the present invention. In FIG. 2,waveform 22 illustrates the total or integral of the uncorrectedillumination energy or flux along the width of the rectangularillumination field, illustrated in FIG. 1, along the Y axis orlongitudinal direction. Portion 26 of wafeform 22 illustrates thereduced illumination energy or flux as a result of the non-uniformity16, illustrated in FIG. 1. Waveform 24 illustrates the more uniformillumination energy or flux as a result of inserting selected blades ofthe adjustable slit device 20, illustrated in FIG. 1, into theillumination profile 10. It should be noted that the energy level orflux along the Y direction or longitudinal length of the rectangularillumination filed or slit 12, illustrated in FIG. 1, is more constantor uniform. This results in a desirable more uniform exposure of aphotosensitive resist covered substrate.

FIG. 3 generally illustrates an embodiment of the present invention. Aframe 28 has an upper support 30 and a lower support 32. Upper support30 has upper bores 36 therein. Lower support 32 has lower bores 38therein. Push rods 34 are placed within the respective bores 36 and 38.The bores 36 and 38 have a diameter of sufficient size to permit pushrods 34 to slide therein. One end of each of the push rods 34 isthreaded and extends through frame bores 40 in frame 28. Placed betweenthe lower support 32 and a portion of the frame 26 are placed nuts 39threaded onto the threaded portion 35 of push rods 34. The other ends ofpush rods 34 are attached to a connecting link 44 by link pivot pins 42.The other end of connecting link 44 is attached to one end of a blade 48by blade pivot pins 46. Therefore, one push rod 44 is coupled to eachend of the blades 48 with the exception of end blades 50. End blades 50have one end coupled to a push rod 44 and the other end coupled to frame28 with end pivot pin 51. The end blades 50 may have slots 53 therein.The blades 48 are generally or substantially rectangular in shape, buthave two corners with a radius. The radius is substantially equal to thedistance between the pivot pin 46 and the edge of the blade 48.Extension supports 52 are attached so as to slide along the side offrame 28. The extension supports 52 may be attached to a rigid supportstructure, not shown. Set screws 54 are used to secure the slidingextension supports 52. The extension supports 54 are used to move orraise the entire frame 28, including the row of blades 48, in unison orall at once. This permits the row of blades 48 to be moved up or downinto a predetermined position without individually moving the blades 48.This may be done for initial positioning or to quickly move the bladeswhen a large adjustment is needed.

FIG. 4 more clearly illustrates a portion of the adjustable slit deviceincluding several of the blades 48 and their motion. Radius 56 on thecorners of the blades 48 provide a smooth transition between adjacentblades 48. A saw tooth shape may also be formed at the intersection 58of two blades 48. Additionally, one end of each blade 48 may have a slot60 therein. The slot 60 may be positioned at every other blade pivot pin46.

The operation of the device can readily be appreciated with reference toFIGS. 3 and 4. Adjustments to the illumination energy are made by movingpush rods 34 which displace blades 48 selectively into the illuminationenergy or flux. Push rods 34 are independently adjusted by turning nuts39. As the push rods 34 are caused to move up and down within the upperand lower supports 30 and 32 the respective blades are moved. Links 44attached to the blades 48 provide lateral or sideways flexibility. Theflexibility is needed due to the nominal differential sideways movementof the spacing between push rods 34 when the blades 48 are moved out ofa straight line. Without this flexibility of sideways movement unduestress or stain may be placed on the push rods 34 or the blades 48. Theslots 60 in blades 48 additionally help reduce any stress or strain dueto the movement of blades 48. Slots 53 may also be placed in the ends50. The flexibility may also be provided by attaching links 44 with aflexure rather than a pivot pin 42. Additionally, a flexure may be usedrather than the rigid link 44 to provide sideways flexibility. Iffrictional forces are not adequate to securely hold the blades 48 inposition, one end of the line of blades may be spring loaded so that apredetermined tension or bias is applied to the blades 48.

It should be appreciated that while the push rods 34 have beenillustrated to be moved with a threaded portion and a nut any othersuitable means or device for moving the push rods 34 is possible, suchas other mechanical or electromechanical means well known to thoseskilled in the art. Additionally, other threaded or screw type push rodsdesigns could be easily adapted to move the blades 48 in practicing thepresent invention. While the resolution of the adjustable slit device isonly dependent on the number of individual blades 48 used, generally asfew as approximately fifteen individual blades 48 have been used withmuch success.

FIGS. 5A and 5B illustrate a partial cross section of another embodimentof the present invention. FIGS. 5A and 5B illustrate anotherconstruction for one of the push rods that may be used in practicing thepresent invention. It should be appreciated and understood that FIGS. 5Aand 5B only illustrate one of the push rods in which there would be anumber or a plurality of push rods with a number or plurality of bladessimilar to that illustrated in FIGS. 1 and FIG. 3. In FIGS. 5A and 5B,the blades 148 are pivotally connected together with a blade pivot pin146. The pivot pin 146 extends through a push rod threaded extension137. The push rod extension 137 is bifurcated so that the blades 148 canfit there between. The push rod extension 137 has a longitudinal bore141 and internal threads 139 therein. The push rod 134 has externalthreads 135 thereon. The push rod 134 also has a reduced diameter 131into which is positioned a support 130. The reduced diameter 131prevents the push rod 134 from moving axially. Circumscribing the lowerhalf of the push rod 130 is a helical spring 162. The helical spring 162is confined at one end by the support 130 and at the other by a springstop 164 attached to the push rod 134. On one end of the push rod 134may be placed a knob 166. Placed within the longitudinal bore 141 of thepush rod threaded extension 137 is at least one flexure 144. The flexure144 is attached to the push rod extension 137 with a pin 146 placedthrough slots 160 in the blades 148. Slots 160 are elongated holesplaced within the blades 148. Not all blades 148 need have elongatedholes or slots 160. The other end of the flexure 144 is pinned to thepush rod threaded extension 137 with pin 142. The flexure 144 provides aflexible link which permits lateral or sideways movement of the blades148. Accordingly, the blades 148 are flexibly linked to the push rodthreaded extension 137 which is threaded into the push rod 134. FIG. 5Bis a partial cross-section illustrating another view of the push rodthreaded extension 137 that is turned or rotated approximately ninetydegrees from the view illustrated in FIG. 5A. In FIG. 5B, the flexures144 are more clearly illustrated. FIG. 5B illustrates two flexures, oneon each side of the blades 148. However, it should be appreciated thatonly one flexure is needed, although in some applications two may bepreferable. The flexures 144 may also have a rectangular cross-sectionsuch that sideways or lateral movement of the row of blades is permittedand movement out of the plane of the row of blades is limited by thelarger lengthwise dimension of the rectangular cross-section of theflexure 144.

The operation of the embodiment illustrated in FIGS. 5A and FIG. 5Bshould readily be appreciated. As knob 166 is turned such that the pushrod 134 is rotated in either direction as indicated by arrow 168, thepush rod threaded extension 137 is caused to move up and down in thedirection illustrated by arrow 170. Accordingly, the ends of the blades148 are caused to move up and down. Thereby, the blades 148 are insertedor removed from the illumination, illustrated generally as 10 in FIG. 1.Spring 162 places a slight tension on the external threads 135 andinternal threads 139 which prevent any unintentional turning, as well astake up any play that may exist. When a plurality of push rods are usedin a system with a plurality of blades 148 linked together, the movingup and down of the push rods 134 causes the blades 148 to move up anddown, results in sideways or lateral forces being placed on the pins 146attaching the blades 148 together. Much of these lateral or sidewaysforces are compensated for by the movement of flexure 144 laterally orsideways. The embodiment, illustrated in FIGS. 5A and 5B, is thereforesimilar to that illustrated in FIGS. 3 and 4. However, the rigid links44, illustrated in FIGS. 3 and 4, are replaced with flexures 144. Boththe link, 44 illustrated in FIGS. 3 and 4 and the flexure 144,illustrated in FIGS. 5A and 5B, are a type of link in that they bothconnect or couple the blades 48 and 148 to the push rods 34 and 134 toperform the same function or purpose in providing sideways or lateralmovement as the plurality of blades 48 or 148 are moved into and out ofthe illumination, as illustrated in FIG. 1.

FIG. 6 is a perspective view illustrating the present invention in anassembled form that may be placed in a lithography tool. Frame 228 isattached onto a mount 274. A plurality of push rods 234, in thisembodiment fifteen, are held in position by frame 228. Threaded onto oneend of the plurality of push rods 234 are a plurality of push rodthreaded extensions 237. Each push rod extension 234 is connected to twoblades 248 by a blade pivot pin 246. The blade pivot pins 246 alsoattach one end of a link, not shown, to the push rod threaded extensions237 and blades 248. The link may be a rigid link or a flexible link,such as a flexure. Link pivot pins 248 attach the other end of the link,not shown, to the push rod threaded extensions 246. The structure isvery similar to that illustrated in FIGS. 5A and 5B. Each end of the rowof linked blades 248 is attached to an extension support 252. Extensionsupports 252 slide within frame 228 and are locked into position withset screw 254. Extension supports 252 permit the row of linked blades248 to be raised in a group. This facilitates initial positioning of therow of linked blades 248. Springs 262 are placed around each push rod34. At the other end of the push rods 234 are placed knobs 266. Theknobs 266 are used to individually turn the push rods 234 causing therespective blades 248 to move up and down, or into and out of arectangular illumination field, not shown. A stationary shield 272 maybe placed near the row of blades 248.

In some applications it may be desirable to provide a predeterminednon-uniform exposure to a photosensitive resist covered substrate orwafer. For example this may be desired when features to be imaged on thephotosensitive resist covered substrate have different line widths.These different line widths may be at different locations along thelongitudinal lengths of the illumination field. Normally, it isdesirable to keep the uniformity along the length of the rectangularillumination field constant. However, when a variety of different linewidths are desired to be imaged it is advantageous to very the exposuredose as a function of the line width. A constant line width to exposuredose ratio can be maintained to improve imaging and system performance.That is when a line width is relatively wide the exposure dose isincreased at the corresponding longitudinal position in the illuminationfield, and when a line width is relatively narrow the exposure dose isdecreased at the corresponding longitudinal position in the illuminationfield. The preferred or corrected exposure dose is independent oforientation of the feature, for example if the line is oriented in avertical or horizontal direction. The increasing or decreasing of anexposure dose along the longitudinal length of an illumination field iseasily accomplished with the device of the present invention.Simulations have indicated that such a method of correction will beindependent of feature size and type over the range of linear responseof a system. Additionally, bias between group and isolated features willnot be affected.

FIG. 7 is a block diagram illustrating a method of the presentinvention. Box 310 represents the method step of inserting a pluralityof linked push rods into an illumination filed. The linked push rods maycomprise a device as illustrated in FIGS. 3-6. Box 312 represents themethod step of determining a preferred exposure dose as a function ofline width of a feature to be imaged. The preferred exposure dose caneasily be calculated based on well known techniques and may considersuch variables as type of resist, substrate material, illuminationenergy, illumination wavelength, scanning speed, among others. Thecalculations may be performed by a computer, or obtained from a look-uptable providing exposure dose as a function of line width. The preferredexposure dose may even be obtained by actual experimental results or aseries of text exposures. Box 314 represents the step of adjusting theindividual linked push rods to block a predetermined amount ofelectromagnetic readiation. This provides a predetermined exposure doseat predetermined positions along the length of the illumination field.Box 318 represents the step of exposing a photosensitive resist coveredsubstrate by scanning the illumination field over the substrate. Inpracticing the method of the present invention the exposure dose along alongitudinal length of the illumination field is adjusted as a functionof the line width of the pattern on the reticle at a correspondinglocation. As the line widths vary on the reticle corresponding locationsalong the illumination filed are adjusted to obtain a desired oroptimized exposure dose. The adjustments in exposure dose may be madeautomatically by motors attached to the linked push rods. Theadjustments in the exposure dose of the illumination field are made foreach reticle used, and can be easily changed for each different reticle.

In another embodiment of the present invention different feature typesof patterns are imaged separately to optimize formation of an image on aphotosensitive substrate, such as a photosensitive resist or photoresistcovered wafer. FIGS. 8A and 8B illustrate reticles having differentfeature types. A vertical reticle 410 is illustrated having a verticalfeature type 412 thereon. A horizontal reticle 412 is illustrated havinga horizontal feature type 416 thereon. The vertical feature type 412illustrated on vertical reticle 410 is greatly simplified forillustration purposes. It should be appreciated that the verticalreticle 410 is composed of a large number of elements to be imaged ontoa photosensitive substrate or photoresist covered substrate. However,the vertical reticle 412 contains thereon predominantly vertical featuretypes, such as lines oriented primarily in the vertical direction.Similarly, FIG. 8B illustrates a horizontal reticle 414 havinghorizontal feature types 416 thereon. Similar to the vertical reticle410, it should be appreciated that the horizontal reticle 414 hasthereon a relatively large number of horizontal feature types 416, suchas lines that are predominantly oriented in the horizontal direction.The vertical feature types 412 being orthogonal or at right angles tothe horizontal feature types 416. The use of the terms horizontal andvertical are relative and are used for convenience. The horizontal andvertical orientations of the feature types may be rotated with referenceto the reticle. This is illustrated in FIGS. 9A and 9B. In FIG. 9A afirst reticle 418 has a first feature type 420 thereon. In FIG. 9B asecond reticle 422 has a second feature type 424 thereon. The first andsecond feature types 420 and 424 may be oriented at different anglesrelative to the reticle orientation. However, preferably the firstfeature type 420 is orthogonal relative to the second feature type 424.

FIG. 10 schematically illustrates a cross section of a portion of asemiconductor device manufactured with lithographic techniques. Placedon a substrate 426 are a first line 428 and a second line 430. Only onelayer is illustrated in FIG. 10, however, typically many differentlayers may be added depending upon the semiconductor device beingmanufactured. The first line 428 has a lateral width W and a spacebetween adjacent lines. However, the term line width may also be used toindicate the space between adjacent lines 428 and 430.

FIG. 11 is a plan view of the first line 428 formed on the substrate 426illustrated in FIG. 10. The first line 428 has a first edge 432 and asecond edge 434. It is generally desirable that the edges 432 and 434are straight such that the line width is uniform. With very small linewidths, in the order of 120 nanometers, it is often very difficult toobtain a uniform line width and therefore straight edges with existinglithographic techniques. Accordingly, some widths along the line may berelatively narrow, such as width W', and some widths along the line 428may be relatively wide, such as width W". This embodiment of the presentinvention helps to reduce the variation in line width irrespective offeature type.

FIG. 12 illustrates in block diagram form a preferred embodiment of thepresent invention for adjusting the illumination field, and thereforeexposure, in order to achieve a more uniform line width using differentselected feature types. Box 436 represents the step of selecting asingle feature type to be optimized, for example, a feature type ofeither a vertical or horizontal orientation. Box 438 represents the stepof using a reticle with the selected feature type that has been selectedto be optimized. Box 448 represents the step of adjusting anillumination field to optimize exposure to achieve a more uniform linewidth having little variation. The step of adjusting the illuminationfield is preferable performed using the adjustable slit of the presentinvention. For example, when a line width is relatively narrow,electromagnetic illumination is blocked by the adjustable slit beinginserted into the illumination field at the location of the narrowportion of the line width, reducing exposure and thereby increasing theresulting line width after processing. Where a line width is relativelywide, the adjustable slit is adjusted to increase the electromagneticradiation at the location of the wide portion of the line width,increasing exposure resulting in a reduction in line width at thelocation of the relatively wide line width. Box 442 represents the stepof exposing a photosensitive substrate with the dose resulting from theadjustment illumination field. As a result, line width uniformity isgreatly increased. The method illustrated in FIG. 12 is then repeatedfor any number of desired feature types. However, it is preferable thatonly two selected feature types, which are orthogonal, are selected. Thetwo selected feature types preferably have a vertical and horizontalorientation and are orthogonal, but may have any angular orientation. Bypracticing the present invention, very small elements or features withline widths in the order of 120 nanometers may be adjusted by as much as50% of the line width.

The present invention is particularly suitable for deep ultraviolet stepand scan lithography systems. In a scanning system, various variables orfactors may result in the lithographic tool to have a particularsignature. This signature may result in the imaging or printing of linewidths that vary. The present invention uses adjustments in theillumination field or slot to compensate for the signature of thelithographic tool. However, different feature types may need differentadjustments to optimize the resulting line width. The line widthvariation as a function of position along the illumination field or slotmay vary depending upon different feature type orientations. Thisimplies different focal plane shift with features oriented in variousorientations. Generally, this difference is represented as a combinationof common mode focal plane shift and differential mode focal planeshift. In common mode focal plane shift, adjustments may be made suchthat both feature types, horizontal and vertical, are similarly affectedor adjusted in common. Differential mode focal plane shift results ineach feature type, horizontal and vertical, being affected separately.Common mode focal plane shift results from optical field aperture,reticle/stage flatness and other variables. Differential mode focalplane shift result from astigmatism and horizontal vertical bias. Whilethe use of different reticles having different feature types may resultin additional processing time, the additional processing time may onlybe necessary for critical layers which, in practice, consist of a smallpercentage of the total layers needed to fabricate a device. Therefore,additional processing time should be relatively small. In practicing themethod of this embodiment of the present invention, the line widthvariations are measured at locations along the length of theillumination field or slot for each feature type requiring line widthcorrection. This may be initially performed by any conventionaltechnique such as a test pattern with measurements being taken. Themeasured required line width variation correction is transformed into arequired dose profile for each feature type. This dose profile is usedto generate a required adjustable slit setting for a line widthvariation correction for each feature type. The method of thisembodiment of the present invention may utilize the adjustable slitpreviously disclosed or any other equivalent or similar structure tovary the illumination energy along an illumination field or slot. Amultiple exposure technique is used to expose the photosensitivesubstrate multiple times, with each exposure using a differnt reticlewith a single feature type. During each exposure, the illumination fieldis adjusted with an adjustable slit in accordance with the measured orcalculated line width variation correction for each feature type.Generally, two reticles are used resulting in two different exposures.Horizontal and vertical bias is corrected, with each reticle having theparticular feature type only. Reticle design may require insertion ofspecial end of line features to allow for good stitching of the featuresin different orientations where they need to be butted up against eachother.

The adjustable slit may be manually changed each time a differentfeature type is exposed with a different reticle. However, theadjustable slit can easily be automated with suitable well known motors,sensors, and software control to set the different positions of theadjustable slit. Accordingly, the adjustable slit may be set rapidly andaccurately, significantly reducing the time required to adjust the slitand thereby the illumination field energy. In order to reduce reticleswitching time, reticles having the two different feature types,horizontal and vertical, may be placed on the same substrate. This maybe possible with conventional reticles with no field size lost.

Accordingly, this embodiment of the present invention greatly enhancessystem performance in a scanning lithographic tool, and makes possiblethe controlled manufacture of very fine feature sizes or line widths.

Accordingly, is should be appreciated that the present invention greatlyfacilitates the adjustment of a rectangular illumination field or slitand provides more uniform illumination energy that is particularlyuseful in a scanning lithography system. The linked set of bladesprovides smooth transitions greatly enhancing the ability to adjust theillumination energy for providing uniformity in exposure of aphotosensitive resist covered substrate, such as a semiconductor wafer.Additionally, the process or method of adjusting the illumination fieldaccording to an embodiment of the present invention greatly facilitatescontrolling line widths of elements having relatively small featuresizes.

FIGS. 13-17 illustrate another embodiment of the present invention thatdynamically adjust the width of the illumination field to controlexposure dose over local areas of the exposed photosensitive substrate.Accordingly, local areas of line width variances are compensated forduring scanning of the dynamically adjustable illumination field. Inthis embodiment of the present invention, the adjustable slit iscontrolled dynamically during the scanning operation to improve imagingperformance and reduce line width variations. The illumination intensityor energy delivered to any local area within the exposure field may bevaried by dynamically adjusting the width of the adjustable slit.

FIG. 13 is a schematic perspective view illustrating this embodiment ofthe present invention. A substantially rectangular illumination field510 has a maximum width of W_(max) and a minimum width of W_(min). Anillumination adjuster 520 extends along a longitudinal length along oneside of the substantially rectangular illumination field 510. While theillumination field 510 is illustrated as rectangular, other shapes maybe used. The illumination adjuster 520 may be made from a plurality ofadjustable lengths as previously illustrated and described. Theillumination adjuster 520 forms a predetermined contour 524 along alongitudinal length of the illumination field 510. Preferably, theillumination contour 524 may be adjusted in relatively small incrementsin both the longitudinal and lateral direction. In the longitudinaldirection, increments as small as one millimeter are easily achievedwith much smaller dimensions achievable in the lateral direction. Thelongitudinal direction is only limited by the size of the blade insertedinto the illumination field. The lateral or width dimension is onlylimited to the movement of the drive actuator. An adjustable contourdrive 533 may be comprised of a plurality of solenoids, or theirequivalent, to selectively or predeterminedly adjust or change theadjustable contour 524. Accordingly, the substantially rectangularillumination field 510 may be adjusted along its length in apredetermined way within the range of the maximum width is W_(max) andthe minimum width is W_(min). The adjustable illumination field 510 iscaused to scan a reticle 537 placed on a stage 535 by the stage control539. Arrow 518 represents the scanning direction X. An adjustable slitcontrol 541 is coupled to the illumination adjuster control 533 causingthe adjustable contour to vary in a predetermined way, modifying theillumination field along a longitudinal length. The exposure doseprovided to a photosensitive substrate during scanning can thereby bemodified or adjusted to compensate for variations in line width. Anexposure calculator 543 provides information to the adjustable slitcontrol 541 to obtain a desired exposure dose to enhance the image ofthe reticle 537 reproduced on the photosensitive substrate.

In operation, as the illumination field 510 is scanned across thereticle 537, the width of the illumination field 510 is continuouslyadjusted and controlled by the adjustable slit control 541 based oninformation provided by the exposure calculator 543. Accordingly, theillumination energy or illumination intensity provided to thephotosensitive substrate is dynamically adjusted over various localareas or regions to compensate for any variations in line width,non-uniformity in the illumination field, variations or irregularitiesin the reticle which may effect image quality on the photosensitivesubstrate, and particularities in the projection optics. All of thevariables associated with a particular photolithographic system or toolare generally referred to as a signature. A system signature can readilybe determined by exposure of test reticles to obtain the requiredindividualized adaptations to an exposure dose in order to maintain orprovide optimum reproduction of an image on a photosensitive substrate.In addition to the various variables associated with an advancedphotolithographic system or tool, changes may occur over time which mustbe compensated for. For example, the illumination source may fluctuate,optics may age reducing their imaging properties, and reticles may vary.The dynamically adjustable slit of this embodiment of the presentinvention makes possible the fine adjustments to the illumination fieldand resulting exposure dose during a scanning exposure enhancing imagereproduction on a photosensitive substrate. The adjusted or correctedexposure dose is readily calculable based upon the measured line widthsof an exposed and processed substrate based upon exposure of a testreticle.

FIG. 14 schematically illustrates various local areas of line widthvariance or non-uniformity 545 on a processed photosensitive substrate560. These local areas of line width variance or non-uniformity 545 maybe caused by any one of a number of variables that may be known orunknown. However, the local areas in line with variance 545 are usuallydetermined and measured by exposing a test reticle. After determiningtheir location and measured variances, a corrected exposure dose may becalculated by known techniques.

FIG. 15 graphically illustrates the illumination energy provided to aphotosensitive substrate utilizing the adjustable illumination field ofthe present invention. Under normal or ideal circumstances theillumination intensity or exposure dose would be uniform. However, theillumination energy 521 may be adjusted at various local areas so as toimprove the quality of the image reproduced or printed. For example,location 515 may have an increase in illumination energy or exposuredose in order to reduce a line width, when a positive resist isutilized, on a photosensitive substrate. Similarly, location 519represent an increase in illumination energy or intensity at anotherlocal area. Location 517 represent a decrease in illumination intensityor energy in another local area. It should be appreciated that byutilizing the adjustable slit of the present invention in a dynamic wayduring a scanning operation, any local area may have the illuminationenergy or exposure dose adjusted to improve image quality. Patternreproduction, or printing.

FIG. 16 is a graph illustrating a photosensitive resist or photoresistexposure transfer or response function 532 for a positive photoresist.For a positive photoresist, increasing the exposure or dose will resultin a decrease in line width, opaque line on a reticle, of the resultingprocessed photoresist covered substrate, and decreasing the exposure ordose will result in an increased line width in the resulting processedphotoresist covered substrate. Waveform 534 illustrates thisrelationship; that is, increasing the exposure or dose will result in anincrease in line width of the resulting processed photoresist coveredsubstrate, and decreasing the exposure or dose will result in adecreased line width in the resulting processed photoresist coveredsubstrate. A nominal center point 538 illustrates a dose d that willresult in a width W for a particular photoresist. Point 536 represents areduced dose d⁻ resulting in an increased line width W⁺. Point 540represents an increased dose d⁺ resulting in a reduced or decreased linewidth W⁻. Accordingly, even though a predetermined line width W on areticle is imaged onto a photosensitive substrate, the resultingprocessed line width may vary as a function of dose or exposure. As aresult, based on the photolithographic tool signature and the responsefunction, a corrected local exposure dose can be determined tosubstantially reduce line width variation in the local area.

FIG. 17 illustrates, in block diagram form, the embodiment of thepresent invention for dynamically adjusting the illumination field, andtherefore, exposure dose in order to more accurately reproduce the imageof a reticle onto a photosensitive substrate. Box 550 represents thestep or act of determining the local locations of line width variationsresulting from the projection of an image of a reticle with projectionoptics. Box 552 represents the step or act of calculating a correctedexposure dose to compensate for the line width variations. Thiscalculation may be based upon the photoresist response function asillustrated in FIG. 16 for the particular photoresist used. Box 554represents the step or act of calculating a reduced illumination fieldwidth to obtain the corrected exposure dose. This may easily bedetermined based upon the reduction of area of the illumination fieldand resulting illumination energy delivered to the photosensitivesubstrate. Box 556 represents the step or act of dynamically adjustingthe width along a length of an illumination field while the illuminationfield is scanned over a reticle with the image of the reticle projectedonto a photosensitive substrate. By practicing the above steps or actsaccording to this embodiment of the present invention, the image orpattern of a reticle reproduced on a photosensitive substrate is greatlyimproved.

FIG. 18 graphically illustrates a photolithographic system or toolcapable of locally adjusting the exposure dose provided to aphotosensitive substrate. An illumination source 562 projects the imageof a reticle 537 held in a reticle stage 535 onto a photosensitivesubstrate 560. The image of the reticle 537 is projected onto thephotosensitive substrate 560 with projection optics 549. Projectionoptics 549 may have a magnification of less than one. The photosensitivesubstrate 560 is placed on a wafer stage 558. The reticle 537 is placedin a reticle stage 535. The reticle stage 535 and the wafer stage 558may be moved in parallel planes so as to provide parallel scanning ofthe illumination field 510 in the direction indicated by arrow 518. Thereticle stage 535 and the wafer stage 558 are synchronized or controlledby stage control 539. Between the projection optics and the reticle 537is positioned the illumination adjuster 520 providing the desiredillumination field 510. The illumination adjuster 520 is adjusted by theillumination adjuster control 533. Accordingly, the width of asubstantially rectangular illumination field 510 is controlled, asillustrated in FIG. 13. Data storage 547 stores data related to theprojection optics, photoresist, reticle, and any other informationeffecting image quality or pattern reproduction. The data in datastorage 547 is provided to an exposure calculator 543, which calculatesan optimum exposure based on well known techniques, including thephotoresist response function illustrated in FIG. 16. The exposurecalculator 543 provides calculated information on the preferred oroptimized exposure dose to adjustable slit control 541. The adjustableslit control 541 directs the illumination adjuster control 533 to drivethe illumination adjuster 520, resulting in dynamically modifying thewidth of the illumination field 510 as the reticle 537 and thephotosensitive substrate 560 are scanned in the direction indicated byarrow 518.

The embodiment of the present invention illustrated in FIGS. 13-18facilitate improved performance of a photolithographic system or tool.By dynamically adjusting the illumination field width and therefore theillumination energy or exposure dose provided to the photosensitivesubstrate dynamically while scanning, local areas of variance may becompensated for. This controlling of the illumination intensity orillumination energy longitudinally along the slot or slit position andat preselected times during the scanning operation, makes possible thecontrol of the illumination intensity which may be used to correct forat least two factors. The first fact being reticle blank variance intransmission with position on the reticle, and the second factor beingany pattern of local variance from normal, reticle pattern print biasederrors. The appropriate adjustable elements are driven in or out thecorrect amount to change the illumination level locally the exact amountneeded to compensate for the local deviations. The electronics or drivesystems to control the adjustable elements are straight forward and canbe implemented in many ways well known to those skilled in the art. Thenecessary error signal corrections may be generated in many ways.Preferably, a one off calibration procedure is used. Here, a reticle tobe used is printed on a specific photolithographic tool. The printedline width in the relevant local areas are examined and a line widtherror map generated. From lithographic knowledge of the sensitivity ofline width to exposure, this map is turned into an exposure mapindicating the difference in exposure required to obtain the desiredline width. From this lithographic knowledge and knowledge of thesystem, the exposure map is transferred to an actuator position map overthe whole extent of the reticle and corresponding photosensitive resistsubstrate. The relevant error correction information is stored in asystem computer memory ready to be used when a particular reticle isused for printing. A library of reticle information may be stored andutilized, depending upon the reticle selected for use.

Accordingly, with this embodiment of the present invention, the exposureof different local areas may be adjusted easily, improving theperformance of the photolithographic system. By providing greatercontrol over the exposure of localized areas, the photolithographicsystem can be optimized, providing better reproduction of the pattern onthe reticle being produced. Accordingly, due to the dynamic adjustmentsof illumination intensity or energy provided for exposure, it may bepossible to use less expensive and complicated illumination systems ormaintain a photolithographic tool in production with reduces maintenancedue to the ability to control quickly and easily the illuminationintensity or energy to compensate for system changes normally requiringdown time.

This embodiment of the present invention is similar to U.S. patentapplication Ser. No. 09/232,756 filed concurrently herewith on Jan. 15,1999 and entitled "Dose Correction For Along Scan Linewidth Variation"invented by Andrew W. McCullough, which is herein incorporated byreference in its entirety.

Although the preferred embodiments have been illustrated and described,it will be apparent to those skilled in the art that variousmodifications may be made without departing from the spirit and scope ofthis invention as set forth in the claims.

What is claimed is:
 1. A method of dynamically controlling anillumination field during a scanning exposure of a photosensitivesubstrate with the image of a reticle having line widthscomprising:determining a location of a line width variation; calculatinga corrected exposure dose to compensate for the line width variation;.calculating a modified illumination field to obtain the correctedexposure dose; and adjusting the illumination field to the modifiedillumination field at the location of the line width variation duringthe scanning exposure, whereby the line width variation is reduced.
 2. Amethod of dynamically controlling an illumination field during ascanning exposure of a photosensitive substrate with the image of areticle having line widths comprising:determining a plurality oflocations of line width variations in a processed photosensitivesubstrate; calculating a corrected exposure dose for each of theplurality of locations to compensate for the line width variations;calculating a modified illumination field to obtain the correctedexposure dose for each of the plurality of locations; and adjusting theillumination field to the modified illumination field at each of theplurality of locations of the line width variations during the scanningexposure, whereby the line width variations are reduced and the image ofthe reticle is more accurately reproduced on the photosensitivesubstrate.
 3. A method of reproducing a pattern on a photosensitivesubstrate comprising:projecting the image of a reticle onto aphotosensitive substrate using a photolithographic system; processingthe photosensitive substrate to obtain a reproduced pattern of thereticle; generating a line width error map identifying local areas werethe line widths of the reproduced pattern vary from the reticle;generating an exposure map indicating differences in exposure requiredto obtain a desired line width at the local areas; generating anactuator position map used to control actuators in an adjustableillumination field for obtaining the differences in exposure indicatedin the exposure map; storing data representing the actuator position mapassociated with the reticle; and using the actuator position map data toexpose photosensitive substrates, whereby an illumination field isdynamically adjusted during a scanning exposure to improve reproductionof the image of the reticle.
 4. An imaging method for use in scanningphotolithography comprising the steps of:determining a plurality oflocations of line width variations on a processed photosensitivesubstrate; calculating an exposure dose required to optimize exposure ofa photosensitive substrate at each of the plurality of locations; andexposing the photosensitive substrate with an image of a reticle basedupon the exposure dose, whereby line width variations of a pattern on areticle are reduced.